docs/conf.py: Active sphinx_selective_exclude extensions.

For modindex_exclude extension, per-port module excludes are also added.
With these changes, it's possible to generate docs for a particular port
devoid of any superfluous and unrelated content, including in indexes and
full-text search - with small caveat: when generating PDF docs after HTML,
or vice-versa cached internal doctree representation (build/*/doctrees/)
must be removed first.